The present invention relates to a process for welding plastic films that are positioned lying flat, one on top of the other. The invention further relates to plastic articles produced by the process and to an apparatus for use in performing the process.
It is known that plastic films, such as sheets and cut-to-size pieces, can be brought into flat-surface contact and then welded. But problems arise in the welding of certain plastic films, for example, films comprising polyolefins (polypropylene, polyethylene and the like) as well as other plastics, due to defects which occur during or after the welding operation. Examples of such defects include undulations of the material next to the weld seams, warping effects, shrinkage effects, flaws and the like.
Processes are known in which the polymer films are subjected to a pretreatment, for example, a conditioning before the welding operation (see German patent specification No. 2,630,817). Furthermore, devices are known for the heat-sealing of thin thermoplastic layers, particularly film tubes, in which a heater band or welding band provided with a separating bar is moved against a flexible, compressible support, and welding is carried out under pressure.
These processes and apparatus are disadvantageous due to difficulties which occur, with the welding or with the material being welded, in the case of film sheets comprising certain types of plastic. In particular, unreliable weld seams occur, and only straight welding is possible; no L-shaped, arcuate, H-shaped or similar separating seams can be achieved. Furthermore, it is known that the separating seams can be easily torn open and damaged after the hot-tool welding.